1. Field of the Invention
The present invention relates to a flow rate control apparatus for a oil-hydraulic pump which is employed suitably in a hydraulic excavator, a hydraulic crane or the like and driven by a rotation force from of a motor. More particularly, the present invention relates to a flow rate control apparatus which controls the flow rate discharged from an oil-hydraulic pump to utilize the output power of a motor without overload applied to the motor, and optimally controls the output flow rate of the pump depending upon a manipulated signal to assure an excellent operation capability to an operator under a high load operation condition of a hydraulic machine system with a hydraulic actuator driven on the basis of the discharged flow rate of the oil-hydraulic pump.
2. Description of the Prior Art
In general, a recently proposed hydraulic driving circuit is designed such that the output power of a motor is utilized to its maximum to improve working efficiency. In many cases, according to such a conventional hydraulic driving circuit, the maximum output P of the motor is previously set in consideration of working and load conditions to significantly reduce undesirable energy loss.
More specifically, a variable capacity oil-hydraulic pump has a discharge flow rate determined from a product of the rotational speed of the motor and the inclination-changed value of the inclined plate in the pump. The flow rate discharged from the pump is thus increased in accordance with the inclination-changed value of the inclined plate in the hydraulic pump.
The hydraulic pump is driven by the motor, and as the torque of the oil-hydraulic pump is larger than the output power of the motor, the motor may be overloaded causing the rotational speed of the motor to drop, possibly resulting in that the motor being stopped of the overload to the motor is applied continuously.
For that reason, a regulator is disposed to adjust the inclination of the inclined plate in the pump so as to limit the input torque. With this regulator arrangement, the input torque of the oil-hydraulic pump is limited in a range of the output power of the motor and the output power of the motor is effectively utilized. More specifically, the regulator receives the pressure feed-back from the pump. As the pressure is gradually increased, the regulator properly limits the discharging flow rate of the pump. On the contrary, as the pressure is decreased, the regulator serves to reduce the flow rate so as to effectively utilize the output power of the motor.
With the construction described above, however, since the hydraulic circuit is employed in order to achieve the principle object thereof, the construction is complicated and, hence, the process of fabricating the circuit is also difficult. Further, a technical limit in the process of fabricating the circuit is present, resulting in the decrease in efficiency of the circuit.
Furthermore, the hydraulic circuit for limiting the output level of the hydraulic pump or a hydraulic circuit having an arrangement discharging a flow rate proportional to the manipulating means such as a lever or pedal may be complicated in structure.
In addition, the hydraulic pump discharges a flow rate proportional to the manipulating means at a lower load condition, while the pump discharges the maximum flow rate regardless of the manipulated variable when the manipulated angle of the inclined plate is gradually changed to a higher load condition. As a result, the operational area available to the operator is relatively reduced and the limitation in operation is also undesirable.
In order to solve the above drawbacks, a control apparatus for load sensing hydraulic driving circuit is proposed in Japanese patent laid-open publication No. 2-275101. With the control apparatus, when the discharging flow rate of oil-hydraulic pump is in a saturated condition, a correction of the total flow rate consumed by a pressure correctable flow rate control valve is executed with a substantially improved manipulation capability. Also, the control apparatus suitably controls the pump without a hunting phenomenon in controlling the pump.